According to the U.S. Census Bureau, the county has a total area of 1,190 square miles (3,100 km2), of which 241 square miles (620 km2) is land and 948 square miles (2,460 km2) (79.7%) is water.[4] It is the third-smallest county in Wisconsin by land area. It is watered by the Milwaukee, Menominee, and Root rivers. The surface is undulating, and the soil calcareous and fertile.[5]

There were 377,729 households, of which 29.5% had children under the age of 18 living with them, 39.0% were married couples living together, 16.3% had a female householder with no husband present, and 40.4% were non-families. 33.0% of all households were made up of individuals and 10.7% had someone living alone who was 65 years of age or older. The average household size was 2.43 and the average family size was 3.13.

In the county, the age distribution was spread out, with 26.4% under the age of 18, 10.5% from 18 to 24, 30.3% from 25 to 44, 20.0% from 45 to 64, and 12.9% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 92.0 males. For every 100 females age 18 and over, there were 88.1 males.

In 2000, the county was the center of the Metropolitan Area, in which the residential pattern of Blacks versus Whites was the most segregated in the country, according to the U.S. Census Bureau.[11]